The CEOs, Randall Stephenson of AT&T and Jeffrey Bewkes of Time Warner, are due to appear Wednesday before a Senate antitrust panel whose Republican chairman and senior Democrat have said the proposed $85.4 billion merger of the communications giants would potentially raise significant antitrust issues. The Senate Judiciary subcommittee on antitrust doesn't have authority to rule on the merger, but members are likely to use their platform to ask questions such as whether the companies would not share Time Warner content with other cable companies or online video providers.